Among the criteria for an aqueous printing ink is its ability to be transferred from the inking roller to a wide variety of substrates, such as corrugated papers and cartons, plastic film, metal foils, newsprint, and so forth. The print should dry quickly; be strong, clear, sharp, resistant to rubbing and scuffing, and resistant to water; and possess high gloss.
It is known that the transfer properties of an aqueous printing ink can be improved by the addition thereto of polyethylene oxide. In U.S. Pat. No. 4,014,833, for example, ink compositions are claimed that contain 0.1-1.5 weight percent of polyethylene oxide having an average molecular weight between 100,000 and 350,000.
When, however, it was attempted to improve the transfer onto corrugated paper stock of a flexographic printing ink based on, e.g., a styrene-acrylic ester-acrylic acid terpolymer latex by adding thereto a polyethylene oxide, the results werer unsatisfactory for commercial use. With additions of about 0.1 to 0.5 weight percent of the polyethylene oxide, the ink transfer was poor, resulting in weak prints with low color intensity. Additions of about 1.0 to 1.5 weight percent of the polyethylene oxide greatly improved the transfer and the print strength was excellent; however, in a fast roll out there was considerable flying of the ink at the edges of the print, giving it a speckled appearance ("misting").